With the increase of internet traffic, an optical communication system in a trunking system requires a greater capacity. When a bit rate per wavelength increases, the chromatic dispersion, polarization mode dispersion and various nonlinear effects on a transmission path lead to waveform distortion, which results in poor quality of information.
FIG. 1 is block diagram of a typical digital coherent receiver. The digital coherent receiver receives an optical signal, and the received optical signal is divided into two mutually orthogonal signals in a polarization state by a polarization splitter. The polarization splitter outputs the polarized optical signals, and the polarized optical signals are mixed with a local-oscillator optical signal via a 90° photomixer. The mixed optical signals are converted into baseband electric signals through a balance photoelectric detector. The photovoltaic-converted electric signals have two signals for each polarization state. Because there is crosstalk between the two polarization states and the polarization states also have rotations after passing through the transmission channel, each of the two polarization states at the receiving end has two orthogonal signals which have no corresponding relation with the transmitting signals. Then, the electric signals are converted into digital signals via an analog-to-digital converter, so that the digital signals may be processed through a digital signal processing technology.
Compared with an incoherent technology, a digital coherent receiving technology has the following advantages: about 3 dB OSNR (optical signal to noise ratio) gain is achieved; an electric equalization technology may be conveniently used to cope with channel variations, reduce the cost, and the like; and a more effective modulation technology and polarization multiplexing technology can be employed to improve the transmission capacity.
In the digital coherent receiver, the equalization of chromatic dispersion and polarization mode dispersion is generally completed by two parts. Firstly, a static dispersion is compensated, wherein a Fast Fourier Transformation (FFT) technology is usually employed to perform a fast frequency domain convolution so as to complete the compensation of the static dispersion; and then residual chromatic dispersion and polarization mode dispersion are compensated, wherein a Finite Impulse Response (FIR) butterfly equalizer is employed for implementing the compensation, and the FIR butterfly filter employs a self-adaptive algorithm to update coefficients so as to track and compensate the polarization mode dispersion dynamically changed with time. The FIR butterfly self-adaptive equalizer has the functions of equalization, matched filtering and phase adjustment. However, when a sampling frequency deviation exists or a sampling phase changing range exceeds a range adjusted by the FIR butterfly self-adaptive equalizer, the FIR butterfly self-adaptive equalizer cannot work normally. Therefore, it is required to place a clock phase recovery device before the FIR butterfly equalizer, for evaluating the phase error of the input time/frequency domain signal, and performing a phase adjustment on the input time/frequency domain signal so as to ensure that a stable and proper sampling phase is provided to the self-adaptive equalizer.
The clock phase recovery methods in the related arts include a square clock recovery algorithm, and a Gardner clock recovery algorithm, and during the process of implementing the present disclosure, it is found that the existing clock phase recovery solution at least has the following defects:
because the differential group delay and the polarization rotation angle of the polarization mode dispersion change with time, the phase of the input time/frequency domain signal would be affected by the polarization mode dispersion; moreover, when the differential group delay is a half period while the polarization angle forms a 45-degree angle with the principal state of polarization at the receiving end, the existing clock phase recovery solution cannot extract the phase error of the input time/frequency domain signal, and at this moment the clock phase recovery cannot be implemented.